Restricted permutations.

Terms

a(0) =6a(1) =9a(2) =12a(3) =18a(4) =27a(5) =42a(6) =66a(7) =105a(8) =168a(9) =270a(10) =435a(11) =702a(12) =1134a(13) =1833a(14) =2964a(15) =4794a(16) =7755a(17) =12546a(18) =20298a(19) =32841a(20) =53136a(21) =85974a(22) =139107a(23) =225078a(24) =364182a(25) =589257a(26) =953436a(27) =1542690a(28) =2496123a(29) =4038810